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Spin-Transport in inhomogeneous Ferromagnets

Final Report Summary - MRHELIMAG (Spin-Transport in inhomogeneous Ferromagnets)

The overall aim of the MR-HELIMAG project is to investigate the magnetoresistance of epitaxial (single-crystal) rare-earth multilayer films. Several rare-earths, for example holmium and dysprosium which have a hexagonal crystal structure form a magnetically ordered state in which the spins in each basal plane are aligned, but the alignment direction shifts along the c-axis and so form a helical antiferromagnetic state. This magnetic configuration can be modified by either magnetic field or temperature into a fully-ordered ferromagnetic state. In order to understand the effect of this helical state on the electrical properties it is necessary for the current flow to be along the c-axis direction. This was achieved within the project by creating nanopillar devices using focused ion beam milling. The attached image shows a typical device structure.

The primary task of the initial stages of the project was to achieve high quality epitaxial films. Although this was eventually achieved, this took longer than originally anticipated and so films were also obtained from a collaborator. Measurements on these samples showed a distinctive correlation of the magnetoresistance with magnetic state of the rare-earth multilayer. In particular we have observed a positive magnetoresistance to the onset of fan-type phases which are intermediate between the helical antiferromagnetic state and the fully aligned ferromagnetic state (see attached figure).

Existing theories relate the spin-dependent scattering mechanism in rare-earth metals to the spin-disorder within the non-collinear magnetic structure; here we have shown that spin correlations arising from the long-range order within these fan-phases are important controlling factors for the magnetic scattering.

The potential impact of this work lies in the improved understanding of the spin transport in rare-earth heterostructures. Within the basic research sector, this knowledge paves the way for the exploration of complex spintronic effects, such as spin-transfer torque, in materials which do not have conventional spin-aligned ferromagnetic order. The rare-earth materials order at temperatures below room temperature and so short-term socio-economic impacts are not expected, but there are materials systems in which equivalent behavior might be explored.

For further information please contact Prof. Mark Blamire ( Department of Materials Science, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS